The advantage of low thrombogenicity without anticoagulation therapy may be outweighed by diminished durability in the second decade following implantation of glutaraldehyde preserved porcine bioprostheses. Hancock mitral valve bioprostheses have been implanted at the National Institutes of Health since July 1970. Eight porcine models were placed in 111 patients during a 41 month period ending December 1974. These patients have been followed annually by the NHLBI Surgery Clinic by annual examination and serial hemodynamic studies. Intrinsic valve failure, defined as structural degeneration of valve tissue and/or stent geometry alteration, in the absence of a history of infection, occurred in 23 patients. The linearized and actuarial incidence of valve failure increased markedly in the ten-fifteen year interval since implantation. The incidence and rate of failure were not related to model type. No predictors of valve failure were found on early postoperative catheterization. Catheterization prior to reoperation demonstrated significant prosthetic stenosis and valvular regurgitation. Explants showed gross leaflet disruption and/or perforation, with variable mixtures of calcification, stent creep, and intracuspal hematomae. Mortality at reoperation was high, subduing our initial enthusiasm for the Hancock mitral bioprosthesis in younger patients, in those who might require eventual anticoagulation, and in those who would present prohibitive operative risks in 8-12 years.